Recently, with a trend toward miniaturization and high-integration of semiconductor devices, EUV (Extreme Ultra Violet) lithography technology using EUV light of 13.5 nm, which is shorter compared to an ArF excimer laser (with wavelength of 193 nm) mainly used in current photolithography technologies of semiconductor devices, is drawing attention as a next generation technology that makes it possible to fabricate a hyperfine pattern of 20 nm or less.
However, a thickness of an EUV resist film is about 50 nm, which is less than half the thickness of an ArF resist film that is now widely used. For this reason, when a film to be etched (a base film, referred to as an etching-target film) is plasma etched by using the EUV resist as a mask, the EUV resist mask is also etched, so that a thickness or line width of the etching-target film cannot be made to a desired value.
Japanese Patent Application Publication No. 2008-505497 discloses a method of performing a plasma etching process by using, as a mask, a resist which has a small thickness and is formed of two layers.
In the method disclosed in the above-cited reference, a pattern is formed by an exposure process on an upper layer of the resist and then a lower layer of the resist is etched with a silicon-containing gas, SiCl4. The silicon-containing gas is deposited on the side wall of the lower layer when the lower layer is etched, and an etching mask of the two-layer resist is formed by realizing anisotropic etching.
In photolithography technologies, an antireflection film is commonly formed since it prevents the dimensional percision of a resist from deteriorating after being developed by reflecting light of a light source off the silicon substrate when the resist film is exposed. The above-described EUV resist film is useful in etching the antireflection film.
However, as disclosed in the above-cited reference, when the etching mask is formed by the anisotropic etching, it is very difficult to precisely adjust a size in the horizontal direction of the etching mask. Therefore, it is difficult to make the line width of the etching-target film be a desired value.